8 August, 1907.] Elcmoiis of Animal PJtysiology. 465 



the artificial clothing of human beings.* When a bird ruffles its feathers 

 in cold weather it merely adds to the enclosed air and this acts as an extra 

 layer of clothing. The same action may be obser\'ed in many mammals. 



Heat is lost from the skin in three ways. First by radiation, in the 

 ■same way as a fire in an old-fashioned fireplace radiates its heat into the 

 room. Secondly by conduction, that is by warming the air with which 

 the surface of the body is in contact ; thus a stove in the middle of a 

 •room not only radiates heat, but also conducts heat by warming the 

 adjacent air. Loss of heat by conduction is great when the air is 

 cold, greater when the air is cold and moist, and greater still when the air is 

 cold, moist, and in rapid movement. The third method is by evaporation 

 •of water or sweat. Sweat glands are found in great number in man and 

 the horse ; very much less in sheep and swine ; and still less in cattle, 

 dogs and cats. In man and the horse the amount of sweat secreted is 

 generally just enough to balance the loss by evaporation so that the sweat 

 just comes to the surface of the sweat pores and no more. When the 

 evaporation is checked by certain conditions of the atmosphere, or when 

 excess of sweat is secreted, as in violent exertion, certain diseases and 

 disorders and nervous disturbances, the sweat accumulates and trickles 

 down the surface, particularly from those regions most abundantly supplied 

 with sweat glands. 



The cooling action of evaporation is familiar in the various devices 

 for keeping drinking water and butter cool in warm weather as by porous 

 clay or damp cloth. Evaporation is greater the drier the air and the 

 quicker the air movement. If the air is saturated with water vapour then 

 no evaporation can take place from a moist surface no matter what the 

 velocity of the air is, or its temperature within the limits that occur 

 naturally. 



Heat is also lost in the lungs and respiratory passages by warming the 

 air breathed in, or inspired, as also by the evaporation of water from the 

 lining membrane of these tracts. This loss of heat from the lungs, etc., 

 plays a more important part in the feebly-sweating animals (cattle, dogs, 

 tcats) than in the sweating (man and horse). 



Some heat is also lost by the warming of food and drink which are 

 -generally cooler than the stomach which they enter. 



The Regulation of Temperature. — We have now to explain how it 

 -comes about that all mammals which are found both in tropical and polar 

 regions are able to maintain a practically constant temperature which 

 -varies very little with species and variety. We shall find that homoio- 

 thenmal animals in cold latitudes have an extra thick coating of fur or 

 •subcutaneous fat to retain the heat. Some such, as the bat, adopt the 

 artifice of becoming poikilothermal during the winter and spending the 

 <;old months sleeping in a sheltered retreat. Then too we are familiar with 

 the seasonal shedding of fur or feathers and the growth of a thicker coat 

 for winter. 



But the most interesting problem is the maintenance of a constant 

 temperature from day to day in each animal despite great changes in heat 

 production and the atmospheric conditions that influence heat loss. Most 

 people are familiar with the principle of the common incubator for hatching 

 eggs. Here there is a mechanical arrangement by which, when the tempera- 

 ture rises a little above what is described, the heat supply is automatically 



*A given volume of flannel or woollen cloth contains a larger volume of air 

 than the same volume of linen or cotton stuff, hence its high value as a clothing 

 material. 



